Paint is normally sold in cylindrical containers which originally were made of metal. Metal paint containers had the limitations, though, of corroding and sustaining dents which could not be removed. More recently, the growth of injection plastic molding has made plastic containers a more desirable solution than metal.
Paint is typically mixed or blended in a machine which either spins the container or inserts an impeller and agitates the fluid directly. Mixing machines create a simple vortex in the center of the container, but the simple vortex is not sufficient for mixing today's paint which requires more random and chaotic fluid dynamics.
The paint industry has experienced changes recently in paint formulation, primarily driven by government regulations requiring removal of solvents. The removal or reduction of solvents has made mixing paint more difficult and time-consuming in stores; the current paint container with a smooth interior wall having mixing ribs does not provide enough agitation to mix paint effectively in a machine mixer without requiring additional time from store personnel. That is, reducing the time of store personnel in mixing paint with a machine at the time of purchase would have a large impact on the efficiency of these store personnel.
Although internal structures in a container to improve mixing of liquids within the container have been known in the art, they have not been common in the paint container industry because they were unnecessary prior to government regulatory changes. Until fairly recently most paint containers were metal. Mass production of mixing ribs in a metal container requires attachment with mechanical means such as rivets, screws and nuts, and can be costly and impractical,
Mixing ribs larger than those in the present invention have a greater tendency to create areas within the container which interfere with usage by consumers with implements such as paint brushes and stirrers. Larger mixing ribs can also prevent the liquid from being mixed sufficiently. The larger the mixing ribs, the more likely they will inhibit mixing by the spinning-type or impeller-type machine, producing an unevenly mixed fluid. Mixing ribs which extend toward the center of the container further than the inner circumference or border of the rim are contrary to the invention herein. An alternative way of stating this is that prior art mixing ribs often extend toward the center of the container a greater distance than does the inner rim border.
The position of the mixing ribs is also important. A mixing rib in prior art can be of many shapes including helical, pyramidal, or planar. Mixing ribs which are not radial or near-radial to the interior cylindrical wall (as specifically defined herein) and the bottom do not create as much agitation as does a radial or near-radial structure of the same height measured from the interior wall to the innermost tip of the mixing ribs. For purposes of delivering a container to the paint industry which is economically feasible, the less plastic used in a container the better, so a smaller structure is highly desirable. A helical mixing rib, for example, in a cylindrical container with a smooth exterior wall requires more plastic material than a radial or near-radial mixing rib of the same height from the interior wall.
Contours which cover substantially all of the internal walls (as opposed to the smooth internal wall with intermittent radial or near-radial mixing ribs of the invention herein) are too complicated to make or require too much raw material to be economically feasible in the mass market.
Therefore, missing from the prior art is the optimal mixing rib configuration (e.g., shape, size, location and quantity) to maximize even mixing, reduce raw material requirements in production and provide easy access with a paint brush for a user. The prior art does not provide an adequate solution for mass production of a plastic container producing sufficient mixing of paint. Given the foregoing design constraints, it is therefore desirable to employ a different design for an injection molded plastic container which will decrease the cost, increase the potential for mixing paint or other liquids, and also be highly practical for a user.